225. Implement Stack using Queues
   My Submissions QuestionEditorial Solution
Total Accepted: 37060 Total Submissions: 121389 Difficulty: Easy
Implement the following operations of a stack using queues.

push(x) -- Push element x onto stack.
pop() -- Removes the element on top of the stack.
top() -- Get the top element.
empty() -- Return whether the stack is empty.
Notes:
You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is empty operations are valid.
Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).
Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

Credits:
Special thanks to @jianchao.li.fighter for adding this problem and all test cases.

Subscribe to see which companies asked this question


class Stack {
public:
    // Push element x onto stack.
    queue<int> q1, q2;
    void push(int x) {
        q1.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        while(q1.size() != 1) {
            int t = q1.front();
            q1.pop();
            q2.push(t);
        }
        q1.pop();
        while(!q2.empty()) {
            int t = q2.front();
            q2.pop();
            q1.push(t);
        }
    }

    // Get the top element.
    int top() {
        return q1.back();
    }

    // Return whether the stack is empty.
    bool empty() {
        return q1.empty();
    }
};